Downhole motor assemblies, such as mud motors, are used to supplement drilling operations by turning fluid power into mechanical torque and applying this torque to a drill bit. The drilling fluid or drilling mud is used to cool and lubricate the drill bit, carry away drilling debris, and provide a mud cake on the walls of the annulus to prevent the hole from sloughing in upon itself or from caving in all together.
One example of a drilling assembly using a mud motor is illustrated in FIGS. 1 and 2. In FIGS. 1 and 2, the downhole assembly includes a motor 11 that is suspended on a string of tubing in the well. The motor 11 is of a progressive cavity type, and has a tubular housing 15 that contains an elastomeric stator 17. The stator 17 is a stationary elastomeric member having cavities 19 throughout its length. A rotor 21 extends through the cavities 19, and rotates as a fluid is passed through the motor 11.
The downhole assembly has a longitudinal axis 35 that coincides with the longitudinal axis of the motor 11. The rotor 21 will orbit eccentrically relative to the axis 35, as indicated by the numeral 37. The amount of lateral deviation from the axis 35 may be on the order of about 3.1 mm to about 6.4 mm (about ⅛ to ¼ inch), for example. The rotor 21 is connected to a connector shaft 39 by a rotor coupling 41. The rotor coupling 41 forms a rigid connection that causes the upper end of the connector shaft 39 to orbit in unison with the lower end of rotor 21. The lower end of the connector shaft 39 connects to a drive shaft coupling 43, which is also a rigid coupling. The drive shaft coupling 43 rotates concentrically on the longitudinal axis 35. The connector shaft 39 will flex along its length because of the orbiting movement of its upper end. The drive shaft coupling 43 is then connected via a drive shaft 45, directly or indirectly, to the drill bit.
In operation, the motor assembly will be assembled and lowered into a well on a string of tubing. Once in place, drilling mud is supplied to the motor 11, causing the rotor 21 to rotate eccentrically. This causes the connector shaft 39 to rotate, which in turn rotates the drive shaft 45 and the drill bit (not shown) connected thereto. The motor 11 will discharge the fluid out the lower end and thence to the drill bit for cooling of the drill bit and removal of drill cuttings, where it flows to the surface. Because of the severe operating and environmental conditions associated with oilfield applications, mud motors may fail due to insufficient adhesion or bonding between materials forming the components of the mud motors.